1. Field of the Invention
The present invention relates to a data communication apparatus, and more particularly to a data communication apparatus having an error retransmission function.
2. Description of the Related Art
Conventionally, when data transmission is carried out through a communication line, errors occur in data at a fixed probability due to the effect of hits, noises, disturbances, or the like occurring in the communication line. In order to assure a fixed level of quality with respect to data which does not virtually include redundancy, unlike the case of analog speech signals, it is necessary to effect coding to detect and correct transmission errors; namely, it is necessary to effect error detection/correction coding.
Coding is effected in the form of adding a redundant data series determined in accordance with a fixed rule on the basis of the relation of that series to a data series which carries information.
Detection of a transmission error is carried out by detecting whether or not that rule is being maintained in a reception data series. After detection of the error, an information data train which includes the error data is retransmitted (ARQ: automatic repeat request), or, instead of retransmission, the detection and correction of the error data are executed by using an error correction code alone on the basis of the disruption of the aforementioned rule (FEC: forward error correction). Incidentally, there are also cases where a hybrid system combining ARQ and FEC is adopted.
In the FEC system, the error code alone is used for the correction of error data, while, in the ARQ system, the code is entrusted with only the task of detecting an error, and then has a proper data train retransmitted, so that a control procedure (protocol) therefor is required. In addition, there are frequently cases where FEC and ARQ are used for two different purposes, the former as a means of improving an error rate of a transmission line and the latter as a means of realizing substantially error-free and highly reliable data communication, including cases where the error rate characteristics of a transmission line are not very certain.
In the ARQ system, the following three systems are used depending on a method of retransmission:
(1) Basic ARQ (Stop-and-Wait, Idle-RQ)
This is a system wherein a confirmed response is returned each time one data block is received, and an ensuing data block is sent after confirmation.
(2) Continuous block transmission ARQ (Go-back-N)
This is a system wherein data blocks are transmitted continuously, and when a response for request for retransmission is given, the process returns to the block in which the error occurred, and an ensuing block N is retransmitted without disturbing the order of the blocks.
(3) Selective retransmission ARQ (Selective Repeat)
This system is a modification of the continuous block transmission ARQ, and is designed to retransmit only an error block.
The foregoing description appears in Data Communication Handbook, compiled by the Electronic Communication Society and published by Ohm, Co., Ltd., published Oct. 30, 1984, first edition and 1st print.
In the above-described systems, an ECM (error correction mode) is currently being studied as an option to the CCITT recommendation on G3.
In the ECM, on the transmitter side, encoded video information is divided at predetermined lengths, and is subjected to HDLC (high level data link control procedure) formatting as frames, and a plurality of frames are transmitted at a time (a maximum of 256 frames are transmitted). Meanwhile, on the receiver side, after reception of the plurality of frames, the numbers of correctly received frames and error frames are transmitted in the form of a bit map (specifically, a signal called PPR) to the transmitter. On the transmitter side, an error frame is retransmitted. If there is no error frame, the receiver sends information to that effect to the transmitter, and the transmitter proceeds with the transmission of an ensuing block. In addition, after the transmitter effects retransmission an n (=1, 2, 3, . . .) number of times, if there is still an error frame, the transmitter can decide whether to continue retransmitting that frame (specifically, retransmission of a CTC signal) or to interrupt the retransmission of that frame and to proceed with the transmission of an ensuing block (specifically, transmission of an EOR signal).
For this reason, there are cases where an error frame remains on the reception side and that error frame is not recorded and reproduced on the reception side.
However, when an error frame remains on the reception side, if an image of the portion of the error frame concerned is not reproduced at all, there are cases where the operator is incapable of knowing that there is an error line. For instance, if one sentence is omitted due to an error, there are cases where it is impossible to know that an error has occurred. Accordingly, if important information is omitted, this can result in a serious problem.
Conventionally, a type of facsimile apparatus is known which is provided with an error retransmission function in which image information is transmitted after being subjected to HDLC formatting. In such a facsimile apparatus, the time duration of sending a flag which is sent immediately before transmission of image information is fixed if encoding of data of one frame or more has been completed. In addition, when if the flag is sent for a fixed period of time, when coding of data of less than a frame has not yet been completed, the flag is sent until encoding of the data of one frame is completed.
Furthermore, after the transmitter transmits a plurality of frames, the transmitter is informed of an error frame from the receiver and effects retransmission. In this case, since the retransmission data has already been encoded, the time duration of sending a flag sent immediately before transmission of the image information is fixed.
When the condition of the line is favorable, it is possible to shorten the communication time if the time duration of sending a flag which is sent immediately before transmission of the image information is short. However, when the condition of the line is aggravated, if the flag transmitting time is fixed, actual data is transmitted before arrangements for a modem are completed (before the adjustment of the modem is completed), so that data cannot be received correctly.
For instance, when the line condition is aggravated and arrangements for the modem of the receiver are not completed unless the receiver receives a flag for 300 ms or more, there is a major disadvantage in that the first frame will not be received for an indefinate period.
In addition, a conventional arrangement is known such that the frame in which an error has occurred is merely retransmitted, and no consideration is paid to error conditions such as the error rate or what sort of frames are experiencing errors. Hence, it has been impossible to say that optimum retransmission has been performed. For that reason, there have been cases where an error repeats during retransmission and, hence, a large amount of time is required in transmission.
In conventional error correction, if an error is not corrected, the transmitter starts to transmit the ensuing information, while the receiver outputs information which is free from error.
Specifically, in the above-described ECM mode, for instance, if an error is not corrected even if the same block is transmitted a plurality of times, it is possible to send a control signal EOR whereby the retransmission of this block is interrupted, and if there is an ensuing block, the ensuing block is transmitted. For example, if the transmitter transmits this EOR, it has not been possible for the receiver to correctly receive the information which was sent before the EOR is transmitted.
Meanwhile, even when the receiver receives the EOR signal, the receiver outputs a frame which has been received correctly.
Accordingly, frequently there have been cases where even if transmission of very important information has been selected and error retransmission has been selected, a received image is output as an image in which an error is present.
Furthermore, there have also been cases where even the frames which have been received correctly in a row are output by the receiver.
Consequently, there is a large drawback in that, although it is desirable for the transmitter side to send an error-free image, an error occurs in the received image and, for example, information which is actually "8" is recorded and reproduced as "0" on the receiver side due to an error, thereby resulting in a large fault.
As described above, there are cases where, when an error frame still appears even if the retransmission of the error frame is effected an n number of times, retransmission of the error frame is suspended and the operation proceeds with the transmission of an ensuing block. At this time, it is necessary for the transmitter, when advancing to the ensuing block, to inform the receiver to that effect by a procedure signal at a transmission speed of 300 bits/sec. Meanwhile, when retransmission of the error frame is to be continued, the error frame is transmitted at a high speed of transmission (e.g. 9,600 b/s, 7,200 b/s, 4,800 b/s, 2,400 b/s). In other words, during the retransmission of the error frame, there are cases where high-speed data (error frame) and low-speed data (300 b/s procedure signal) are transmitted, so that the receiver must be capable of receiving both the high-speed data and the low-speed data during the retransmission of the error frame.
However, when the line condition is aggravated, the low-speed data and the high-speed data may be mistaken for each other, so that there is the major drawback that there is a high probability of communication being incomplete.